A recently published systematic review of evidence on patient and staff experiences, process of care, and clinical and economic outcomes attributable to the Patient-Centered Medical Home (PCMH) concluded that while the PCMH holds promise for improving the experiences of patients and staff and potentially for improving care processes... current evidence is insufficient to determine effects on clinical and most economic outcomes. We propose research to help address the incomplete evidence about the cost of transforming a health care system aligned with medical home principles as well as the change in heath service cost attributable to this transformation. This research will be conducted within Group Health Cooperative, an integrated health care system that provides comprehensive health and preventive services to approximately 450,000 people in an integrated group practice that operates clinics throughout Washington State. The advantage of conducting this research within Group Health is that it is a single organization that both finances health car through a health plan that insures all of its members' health services and directly provides health care through owned and operated primary care clinics and specialty centers. Therefore, we can measure the costs incurred by the health plan of transformation each of its clinics into medical homes as well as the change in costs brought about by this transformation.